1. Technical Field
The invention relates firstly to a digital modulation method which realizes phase modulation or pulse-width modulation or a combination of phase and pulse-width modulation. A digital modulation device suitable for implementing the modulation method is also proposed. The method and the device are suitable in particular for high-frequency (HF) applications. The present invention also relates to a power amplifier, a transmission device and a transmission method.
2. The Prior Art
A modulation method is used to encode a signal for the purpose of wired or wireless transmission. Methods of this kind are essentially known.
For example, FIG. 1 shows a prior art HF Class-D power amplifier. An HF signal modulated in amplitude and phase is fed to the input of a pulse width modulator that encodes the signal into a pulse-width and phase modulated signal (PWM signal). The downstream power amplifier amplifies the PWM signal. Due to the binary signal waveform, the power amplifier may be designed as a nonlinear circuit amplifier, which typically has a high level of energy efficiency. At the output of the power amplifier, the amplified PWM signal is bandpass filtered, as a result of which harmonic frequency components are suppressed, and only the amplified modulated HF signal is sent to the antenna.
FIG. 2 shows a typical implementation of a prior art pulse width modulator. A ramp generator generates a periodic voltage Vref(t) with a triangle waveform. The amplitude and phase modulated HF signal and Vref are fed to a fast comparator. By comparing the ramp with the HF signal, a pulse-width and phase modulated signal is generated.
The pulse width modulator according to FIG. 2 has the advantage of simple implementation. However, it has various serious disadvantages. The first is that the linearity of the modulator, i.e., the accuracy with which the phase and amplitude of the HF signal is converted into the PWM signal, is dependent on the accuracy of the ramp. In the GHz range, it is almost impossible to generate a highly accurate linear ramp. More particularly, the “peaks” of the triangular waveform signal are truncated to a greater or lesser degree due to the finite bandwidth of the electronic components, with the result that the waveform acquires a more sinusoidal than triangular shape, the greater the frequency. This results in nonlinear distortion of phase and amplitude in the modulator. The comparator also switches more or less quickly depending on previous switching operations, because at higher switching frequencies the internal current/voltage compensating actions of the comparator have not fully subsided. Due to these characteristics of the comparator, the PWM signal has additional timing fluctuations (jitter), which can be interpreted as phase and amplitude noise.
Another disadvantage of the pulse-width modulator in FIG. 2 is that very small and very large pulse widths produced by the modulator may cause the power amplifier to not switch on fully, thus leading to nonlinear distortion of phase and amplitude. Yet another disadvantage of this modulator is that phase and amplitude cannot be modulated independently of each other, so it is not possible to compensate separately for amplitude- or phase-specific distortion. Another disadvantage of the modulator is that it is an analog circuit which is greatly affected by fluctuations in processing, voltage and temperature. A further disadvantage is that an HF signal is needed on the input side.
In Frederick H. Raab: “Radio Frequency Pulse Width Modulation”, Transactions on Communications, August 1973, pp. 958-959, a ramp generator is described in which the reference voltage Vref is produced by rectifying the modulated HF signal. At high frequencies in the GHz range, generating the reference signal Vref(t) with a high degree of accuracy becomes difficult in this case as well. More specifically, the rectified signal has a triangular waveform close to the zero crossover, said waveform being rounded and hence distorted by the limited bandwidth of the electronic circuit.
A method and a circuit for pulse-width modulation are known from the publication by J. Keyzer et al.: “Generation of RF Pulsewidth Modulated Microwave Signals Using Delta-Sigma Modulation”, 2002, IEEE MTT-S Digest, pages 397-400, where two delta sigma modulators are used for pulse width modulation, instead of a simple comparator such as the one shown in FIG. 2. A signal to be encoded is represented therein by a phase signal component and an amplitude signal component. The phase signal component is fed to a first delta sigma modulator and the amplitude signal component is fed to a second delta sigma modulator. The output signal of the first delta sigma modulator and an output signal from a pulse generator control a “digital pulse delay modulator”. The output signal from the latter is fed along with the output signal from the second delta sigma modulator to a digital pulse width modulator which generates the pulse-width and amplitude modulated output signal. In the method known from this publication, the phase is modulated first and then the amplitude, i.e. the pulse width of a pulse in the output signal. As a result, the circuit known from this publication is complex in that at least two delta sigma modulators are used and because both the pulse delay modulator and the pulse width modulator must operate at around the frequency of the output signal. Another aspect is that the output signal from such a pulse delay modulator and such a pulse width modulator contains distortions in the case of analog implementation in the high-frequency range.